CI: Add gitlab-ci config.

Similar to how ti was done for gstreamer-rs.
See [1] for more.

[1] https://github.com/sdroege/gstreamer-rs/pull/132

.gitlab-ci.yml

+stages:
+  - "test"
+  - "lint"
+
+.cargo_cache: &cache
+  cache:
+    key: "gst"
+    paths:
+      - ".cargo_cache/"
+
+.cargo_test_template: &cargo_test
+  stage: "test"
+  variables:
+    G_DEBUG: "fatal_warnings"
+    DEPENDENCIES: |
+      curl
+      libssl-dev
+      liborc-0.4-dev
+      libglib2.0-dev
+      libxml2-dev
+      libegl1-mesa
+      libgles2-mesa
+      libgl1-mesa-dri
+      libgl1-mesa-glx
+      libwayland-egl1-mesa
+
+  <<: *cache
+  before_script:
+    - apt-get update -yqq
+    - apt-get install -yqq --no-install-recommends $DEPENDENCIES
+
+    # Only stuff inside the repo directory can be cached
+    # Override the CARGO_HOME variable to force its location
+    - export CARGO_HOME="${PWD}/.cargo_cache"
+
+    - mkdir -p precompiled-gst && cd precompiled-gst
+
+    - curl -L https://people.freedesktop.org/~slomo/gstreamer-1.14.3.tar.gz | tar xz
+    - sed -i "s;prefix=/root/gstreamer;prefix=$PWD/gstreamer;g" $PWD/gstreamer/lib/x86_64-linux-gnu/pkgconfig/*.pc
+    - export PKG_CONFIG_PATH=$PWD/gstreamer/lib/x86_64-linux-gnu/pkgconfig
+    - export GST_PLUGIN_SYSTEM_PATH=$PWD/gstreamer/lib/x86_64-linux-gnu/gstreamer-1.0
+    - export GST_PLUGIN_SCANNER=$PWD/gstreamer/libexec/gstreamer-1.0/gst-plugin-scanner
+    - export PATH=$PATH:$PWD/gstreamer/bin
+    - export LD_LIBRARY_PATH=$PWD/gstreamer/lib/x86_64-linux-gnu:$LD_LIBRARY_PATH
+
+    - rustc --version
+    - cargo --version
+  script:
+    - cargo build --all --color=always
+    - cargo test --all --color=always
+
+
+test stable:
+  # Stable img
+  # https://hub.docker.com/_/rust/
+  image: "rust:slim"
+  <<: *cargo_test
+
+test nightly:
+  # Nightly
+  # https://hub.docker.com/r/rustlang/rust/
+  image: "rustlang/rust:nightly-slim"
+  allow_failure: true
+  <<: *cargo_test
+
+rustfmt:
+  image: "rustlang/rust:nightly-slim"
+  stage: "lint"
+  script:
+    - rustup component add rustfmt-preview
+    - cargo fmt --version
+    - cargo fmt -- --color=always --check